This invention relates to an improved burner and combustion method for use in the melting of glass, metals or other materials. More specifically, this invention relates to a burner and combustion method whereby hot, nearby, and substantially parallel fuel-rich and fuel-lean (oxidant-rich) jets efficiently transport fuel and oxidant to the surface of a material to be heated or melted by direct flame impingement. The method enables one to modify the flow so that either an oxidizing or reducing atmosphere is present at the surface.
The heating and melting of materials such as glass cullet and batch, scrap metal, minerals and ores is important in many industries. Typically, each process has a heat input requirement, a heat distribution requirement and geometrical constraints which hinder the proper placement of a burner and often require the placement of a burner at a much greater distance from the material to be heated or melted than is optimal.
In addition, there are often chemical stability issues relative to heating materials. For example, direct flame impingement on iron or steel usually produces an undesirable oxide coating or oxide slag layer upon melting. Therefore, one would like to heat iron and steel with a reducing gas in contact with the solid or molten iron to minimize oxidation. Other examples of surfaces susceptible to oxidation or reduction include glass forming materials, aluminum, copper, metal alloys, lead, zinc, frit materials, or ceramic materials.
U.S. Pat. No. 5,139,558 to Lauwers discloses the use of oxy-fuel burners located on the furnace roof and aimed at the interface between the batch and the melt in order to increase the melting rate of the glass and to prevent batch materials from entering the upstream zone. However, because the furnace roof to glass melt distance is often greater than typical O.sub.2 -fuel flame length, the thermal efficiency of the process is not adequate for use outside of regenerative or recuperative furnaces.
U.S. Pat. No. 5,643,348 to Shamp et. al attempts to overcome this problem by injecting oxygen and fuel at separate points and producing a large combustion "flame cloud" in the center of the furnace. This approach eliminates the oxy-fuel flame length limitation, but suffers from a safety problem. Where there is no apparent method to maintain reliable ignition of the fuel and oxidant this process could lead to explosions in applications where the ambient conditions are below the autoignition temperature. In addition, the cold fuel or oxidant jet could cause material to solidify and block the nozzle. With fuel-rich and fuel-lean flames of the present invention, the flame propagates back to the inlet nozzle, which keeps the nozzle clear and provides sufficient heat along the entire path of the fuel-rich or fuel-lean flame for ignition.
In U.S. Pat. No. 5,387,100 to Kobayashi the use of rich and lean fuel streams to reduce NO.sub.x emissions is disclosed, however, there is no teaching regarding the interacting of fuel-rich and fuel-lean flames or jets to increase the transfer of heat to the batch or melt. Kobayashi teaches forming fuel-rich and fuel-lean flames, allowing radiation to lower the flame temperature and then allowing the cooler flames to interact without contacting a surface to complete combustion at a lower temperature thereby decreasing NO.sub.x emission. In contrast, in the present invention, radiant losses are minimized prior to interaction with the surface to be heated.
U.S. Pat. No. 5,267,850 to Kobayashi et al. teaches the use of a fuel staged burner to keep the burner cooler. This invention uses similar equipment to produce a fuel rich jet adjacent to fuel lean jet or jets.
U.S. Pat. No. 5,100,313 to Anderson et al. teaches an approach to produce a hot oxygen jet. This oxygen jet is generally used to remove carbon from molten iron to produce steel.
Commonly assigned U.S. patent application Ser. No. 09/384,065 teaches the generation of fuel-rich and fuel-lean flames from separate burners. The flames then interact in gas space in the vicinity of a surface to more efficiently transfer heat to the surface.
The present invention teaches the generation of substantially parallel fuel-rich and fuel-lean jets from a single burner assembly to minimize energy loss during transport through the gas space. The fuel rich and fuel lean jets mix upon collision with a surface to produce a flame adjacent to the surface and provide a shield gas for the surface.
The ability to generate the fuel rich and fuel lean jets from a single burner assembly offers significant advantages over the prior art practice of introducing fuel rich and fuel lean flames from separate sources. Firstly, with separate sources the separate flames, the fuel rich and fuel lean jets, must intersect proximate to the surface to be heated. In the present invention the streams are substantially parallel thus relieving the fixed constraint of burner arrangement with burner to surface distance. This is of importance in situations where the surface of the material to be heated or melted varies. Secondly, the separate fuel rich and fuel lean streams need to intersect at distinct points, therefore, the burners need to be located at defined locations. These locations may be unavailable due to pre-existing structural constraints. The present invention removes the importance of these structural constraints. Thirdly, the alignment of multiple burners to generate intersecting flames at a defined point is difficult. The use of a single burner assembly removes that need.
Therefore, there is a clear and long-standing need for a burner and heating method that would enable one to maximize the amount of heat transferred to a surface of a material to be heated or melted in a variety of furnace geometries while allowing control of the reducing or oxidizing components present at the surface.